A computational fluid dynamics (CFD) analysis showed consistent axial shear at the membrane surface, which became negligible at distances from the membrane surface greater than 0.5 mm. For comparison, CFD analysis of a fully rotating ME system showed local vortices in the continuous phase leading to a variable shear along the axis of the membrane. Using an azimuthally oscillating membrane, oil-in-water emulsions were experimentally produced with a controllable median diameter within the range of 20-120 μm, i.e. a substantial portion of the droplets has a median diameter of 20-120 μm; and a coefficient of variation of droplet size of 8%. The drop size correlated with shear stress at the membrane surface using a force balance. In a single pass of continuous phase, it was possible to achieve high dispersed phase concentrations of 40% v/v.
The production of an emulsion using a microporous membrane developed in popularity as a laboratory study in 1990s, after a publication by Nakashima et al. It became known as membrane emulsification (ME) in which a liquid dispersed phase is injected through the pores of a membrane into a continuous, often cross-flowing, liquid phase. Droplets formed at the pore outlet are detached by the shear created by the flow of the continuous phase on the membrane surface.
Conventionally, in ME the membrane remains stationary and shear stress is applied at the membrane/continuous phase interface to obtain a desired droplet size distribution. Initially, in these systems shear stress was provided by crossflow: higher shear stress provides smaller drops and is obtained by higher continuous phase flow rates, which, in general, leads to lower dispersed phase concentrations of a product for a “single pass” of the continuous phase over the membrane surface.
To overcome this productivity restriction, recirculation of the emulsion can be used. However, when aiming to produce large droplets, recirculation is likely to result in droplet damage within the pump and other fittings present in the system, leading to poor control over the droplet size distribution, limiting the use of this particular ME technique to small emulsion sizes: typically less than 10 μm.
Alternative methods for generating shear at the membrane surface have been described, using stationary membrane systems where shear stress results from stirring, or using pulsed (oscillatory) flow of the continuous phase. Other ME systems have been reported using non-stationary membranes, in which case droplet detachment from the membrane surface is promoted by rotating or vibrating the membrane. In non-stationary membranes, shear stress on the membrane surface is controlled by the speed of membrane rotation, or the frequency and displacement of membrane oscillation/vibration. A major advantage of using a non-stationary mechanically driven membrane is that it “decouples” the control of the drop size by the applied shear from the crossflow of the continuous phase used to remove the product. Hence, in a single pass of continuous phase it is possible to achieve high dispersed phase concentrations of 40% v/v, or more, without recirculation through pumps and fittings.
However, the nature of the mechanically driven membrane does have other consequences. For example, in the case of a fully rotating membrane a centrifugal field will be induced around a rotating membrane. In the most common case of an oil drop being less dense than the surrounding aqueous phase, this will induce flow of the oil drop toward the membrane surface, which is not desirable as the concentration of drops at the membrane surface will increase leading to greater chance of coalescence and wetting of the membrane by the oil phase. Furthermore, having high shear consistently applied in one direction will cause deformation of the emerging oil drops; distorting them in one direction, something highly visible in computational fluid dynamics (CFD) modelling of drops emerging during emulsification, which is again likely to lead to membrane surface wetting and poor drop size control.